1. Field of the Invention
This invention relates to an improvement of a noncontact type tonometer which is designed to measure intraocular pressure of a subject's eye by optically detecting transfiguration of the eye caused by discharging a fluid toward the eye without bringing the tonometer into contact with the eye.
2. Description of the Prior Art
This kind of noncontact type tonometer, for example, as shown in Japanese Patent Application Early Laid-open Publication No. Sho 56-6772, comprises a gas discharging device for transfiguring the eye by discharging gas toward the eye, a light source for illuminating the eye with detecting light, and a corneel transfiguration detecting means for receiving reflected light from the eye to detect the corneel transfiguration of the eye.
If the discharge pressure of the gas gradually increases, a surface of the cornea is transfigured, so that the quantity of the reflected light from the cornea temporarily increases and reaches its peak when the cornea is transfigured into an applenation state (a flat state). If the discharge pressure further increases, the surface of the cornea becomes concave and then the reflected light quantity decreases.
In a conventional noncontact type tonometer, at the time when the quantity of the reflected light from the cornea exceeds a predetermined reference level, namely before the cornea reaches an applanation state, the operation of the gas discharging device is stopped. After the operation of a piston of the gas discharging device is stopped, the discharge pressure increases for a while by inertia of the piston and then decreases. In a case where the intraocular pressure of the eye is normal, since the cornea is transfigured into the applanation state and the peak of reflected light quantity is detected when the pressure within the gas discharging device is increasing because of the inertia, the intraocular pressure can be calculated from the air pressure obtained when the peak is detected.
However, in the conventional tonometer, so to speak, with the expectation that the cornea is transfigured into the applanation state during the rise in pressure caused by the inertia of the piston, the operation of the gas discharging device is stopped. Accordingly, if the intraocular pressure of the eye is much higher than normal and an increase in quantity of the discharge pressure needed for the reflected light quantity to exceed the reference level and reach the applanation state is also larger than normal, the air pressure within the gas discharging device decreases before the cornea is transfigured into the applanation state in spite of the rise in pressure caused by the inertia. For this reason, unfavorably, the peak of the reflected light quantity cannot be detected and the measurement of the intraocular pressure cannot be carried out accurately.